This invention generally relates to reducing power consumption in portable radiotelephones. More specifically, this invention relates to an apparatus and method for reducing cellular telephone transmissions when a variable rate vocoder operates at a minimum encoding rate.
In digital cellular telephone systems, such as code division multiple access (CDMA) cellular systems or time division multiple access (TDMA) cellular systems, a users speech input to a cellular telephone is digitized. Since cellular frequency spectrum is a valuable commodity, compression techniques are employed to reduce the data rate (and therefore the amount of information) necessary to transmit the digitized speech over the cellular airwaves. The digital circuitry used to accomplish at least some of the compression is generally referred to as a vocoder.
A vocoder analyzes an input speech in blocks of time (commonly referred to as frames) and encodes the input using certain vocoder parameters. The vocoder parameters are calculated for each frame. One class of speech coders includes Code Excited Linear Predictive Coding (CELP), Stochastic Coding, and Vector Excited Speech Coding. As an example of the vocoder parameters, in a CELP vocoder, the parameters relate to a Linear Predictive Coding (LPC) filter, a pitch filter, and waveform excitation.
To compress the digitized speech even further, a technique referred to as variable rate vocoding is employed. Variable rate vocoding reduces the data rate for periods of silence in the input speech signal to produce a variable output data (frame) rate.
The use of a variable rate vocoder in the cellular telephone (also referred to as a mobile station) reduces the amount of data that the cellular telephone transmits to increase channel capacity. However, even for encoded frames of the input speech where there is no audio content (e.g. periods of silence), the cellular telephone still transmits the encoded signal. This causes the cellular telephone to use valuable DC battery power more than is necessary. This excessive use of battery power in turn reduces the available talk time for a cellular telephone for any given battery charge. Therefore, there is a need for an apparatus and method to reduce power consumption in a cellular telephone that employs a variable rate vocoder.